Quantitative Fourier Transform Infrared Photoacoustic Spectroscopy of Condensed Phases

The application of Fourier transform techniques to photoacoustic spectroscopy has enabled this method to be extended into the important infrared spectral region between 4000 and 400 cm−1 [1-6]. In making experimental measurements the photoacoustic cell contains the material to be studied and replaces the instrument’s detector. After suitable filtration to remove signal frequencies outside those inherent to the spectral range being studied, the cell output is processed by the standard electronics of the FTIR spectrometer. Because of the selective absorption of the sample in the photoacoustic cell the interferogram has an appearance normally associated with an emission spectrum. The spectral information is therefore distributed over a wide range of path length differences in the interferometer rather than being largely in the "central burst" feature of the interferogram. The selective absorption also means that not all the energy incident on the sample is absorbed and so the central burst is smaller in amplitude than the corresponding feature for a black body absorber. When the data contained in the interferogram is transformed to obtain an absorption spectrum the resulting peak heights are not in the ratios expected due to a number of artifacts introduced by the technique. This paper is concerned with a discussion of these artifacts and procedures that may be employed to remove them so as to obtain a quantitatively correct sample spectrum. Experimental results are presented on some polymeric systems.